This multicenter study is performed to assess outcome in a large cohort of extremely low birth weight (ELBW) infants. One of the primary aims is to track and successfully follow at 18 months of age, more than 80% of babies entered into the Generic Data Base of the Neonatal Research Network with birth weights 401-1000grams. Of special importance is monitoring changes over time associated with changes in care practices and survival. In addition, it is important to assess outcome in infants who have participated in randomized clinical trials performed by the Neonatal Research Network. The results of this study will provide current information on growth, neurodevelopment, and the prevalence of specific morbidities in this population. Additionally, the effects of family socioeconomic status and family stress, as well as the utilization of support services will be characterized.